Practically all communities, whether they are local, regional or national, have a public utility telephone company that connects the individual members of that community. Many subgroups of this community desire to have their own telephone network to communicate amongst themselves. These subgroups also desire to connect their private telephone network to the public utility company so that their members can use their private telephone network to connect to other members of the community. The private telephone network is often referred to as a Private Branch eXchange and the public utility telephone company is referred to as the Public Switched Telephone Network (PSTN).
Many PBXs digitize the telephone conversations and also use digital signals to control the operation of the PBX. This requires the use of a coder/decoder (codec) to code and decode the telephone conversations from analog to digital and vice versa. The codec is also known as a combination of, or just one of, an analog/digital or digital/analog converter. Each individual telephone in a digital PBX includes a codec. The PSTN interface unit connecting the digital PBX to the PSTN also includes a codec for converting the digital telephone conversations of the PBX into analog telephone conversations according to the format of the local PSTN.
Many PSTN's are analog and strictly limit the amount of a telephone conversation that is transferred across the PSTN. This limiting of telephone conversations reduces the quality of the telephone conversation. Codec's in prior art digital PBXs were selected to have the same quality as the local PSTN since any increase in quality of a codec would be eliminated by the local PSTN. Besides the quality level of the local PSTN was often sufficient.
During many telephone conversations, it is often necessary for one of the parties to temporarily stop the conversation and block personal audio from being transmitted. The other party is then put on “hold” and can be made to listen to silence. Some PBXs have structure to receive an external audio signal, such as music or an audio presentation, and transmit this to the other party when the other party has been put on hold. In a digital PBX, a codec is used to convert the analog external audio into digital audio for transmission through the PBX. The quality of the codec used for the external audio would be the same as that used in the PSTN interface unit. In this way, many of the same parts for existing codec's could be used in the external audio codec and the quality would be similar to the quality of the PSTN which was sufficient for the average user.
Many PBXs have been installed in organizations that have a public address (PA) system. It is often desirable to connect the PA system to the PBX so that the PA system can be used from any phone. With a digital PBX a codec is required to convert the digital audio from a phone into analog audio for the PA system. The same quality of codec's that are used to convert digital conversations into analog audio is used for the PA system since the parts are readily available and are designed for use with the digital audio that has been coded at the telephone.
The disadvantage of prior art PBXs is that the quality of music on hold, the quality of audio in a PA system, and the form or format of the digital audio is dictated by the characteristics and requirements of the PSTN.